1. Field of the Invention
This invention resides in the field of propulsion systems, and in particular in the design of nozzles that can be manipulated or transformed during use to adapt to changes encountered in either the propulsion system or the environment.
2. Description of the Prior Art
The thrust produced by a rocket motor tends to change with changes in altitude, environmental conditions, propellant mass flow, and propellant burning rate, and can vary as burning progresses. These changes affect the performance of the motor and of the vehicle driven by the motor, as well as the effectiveness of the vehicle in performing its mission. In some cases, a change in thrust is detrimental to rocket performance, while in others, a change in thrust is desired but may not be achieved quickly enough to the degree that is needed or at the point in time where the change is needed. To illustrate, as propellant supply is gradually depleted, the rate at which combustion gases are generated often decreases, causing a drop in chamber pressure. If this drop occurs during the boost phase of a rocket or at any time when a prolonged period of high-level thrust is needed, the efficiency of the rocket performance will suffer. On the other hand, a decrease in thrust is beneficial in rocket-powered launch vehicles that require high thrust at takeoff due to the large amount of unburnt propellant initially present in the vehicle, and lower thrust during flight. This is particularly true for vehicles that are launched from the earth's surface but whose primary mission is performed at high altitude where the atmosphere is often at high vacuum. The primary mission often requires a high specific impulse (Isp), i.e., a high ratio of thrust to the weight of propellant consumed in a unit of time, which is most readily achieved with a high area ratio, i.e., a high ratio of the area at the nozzle exit to the area at the throat. Nozzles with high area ratios tend to produce relatively low thrust at sea level, however, because of a reverse pressure differential near the nozzle exit that occurs when the wall pressure is below ambient pressure. In supersonic nozzles, one of the most important factors in controlling and maintaining thrust is the nozzle throat, since the pressure drop across the throat directly affects thrust as well as factors contributing to the thrust, such as the chamber pressure.
Numerous constructions have been developed in the history of rocket design for nozzles whose thrust can be varied during flight. Attempts to correct for reverse pressure differentials, for example, have been made by designing nozzles whose nozzle exit area is reduced for launch and then gradually increased during ascent. Nozzles have thus been designed with mechanically adjustable contours, area ratios and lengths. Other methods have included the use of combination-type engines using different propellants at different stages of flight. Kerosene-fueled engines have thus been combined with engines derived from the Space Shuttle Main Engine (SSME) or with hydrogen-fueled engines such as the Russian RD-701 engine. Other examples of combination-type engines are the dual-fuel-dual-expander engine of Beichel, R., U.S. Pat. No. 4,220,001 (issued Sep. 2, 1980), and the dual-thrust rocket motor of Bornstein, L., U.S. Pat. Nos. 4,137,286 (issued Jan. 30, 1979) and 4,223,606 (issued Sep. 23, 1980). The Beichel engine requires a complex nozzle design that incorporates two thrust chambers, while the Bornstein motor achieves dual thrust by using separate sustainer and booster propellant grains in the combustion chamber, together with an igniter and squib that are inserted into the grain itself. Thrust variation has also been achieved by the introduction of secondary combustion gas near the wall of the divergent section of a nozzle, as described by Bulman, M., in U.S. Pat. No. 6,568,171 (issued May 27, 2003).
A still further means of achieving thrust variation is the use of a pintle for varying the effective area of the throat. A pintle is either a tapered or flared body that resides inside the nozzle and is movable along the nozzle axis, partially obstructing the throat and forcing the combustion gas to flow in the annular space between the pintle and the throat wall. With its tapered or flared profile, movement of the pintle by a small distance causes a significant change in the cross section area of the annular space and hence the effective throat area. In a solid propellant system, as burning progresses and the propellant supply rate is reduced, the pintle can be moved in a direction that will reduce the effective throat area in order to maintain combustion efficiency by increasing the pressure in the combustion chamber, although at a lower thrust. A pintle therefore provides versatility to the rocket motor by allowing wide variation in the effective throat area and by being movable at will or programmable to move in either direction at any stage of the rocket propulsion. Pintle actuation can be achieved by a hydraulic drive, a gear drive, or any known mechanism for controlled movement. In many rocket designs, the pintle actuation system is located within the section of the rocket motor through which propulsion gas flows, the actuation system thereby consuming volume that would otherwise be used for gas generation or thrust development. Other limitations of internal pintle actuation systems are that such systems are subject to the high temperatures and other harsh conditions that are present inside the motor and therefore vulnerable to breakdown, and that they are difficult to gain access to when they require maintenance, repair or replacement. The harsh conditions also require that the actuation systems be insulated, thereby adding further weight to the system.